1. Field of the Invention
This invention relates to the production of a trussed rafter with nail plate connections.
2. Description of the Prior Art
Trussed rafters of timber members and nail plates are today produced as follows: the timber members are cut by length and at specific angles with special saws. These saws generally have four blades and the timber moves transversely with respect to the saw. The timber members are assembled in a special jig fixed according to rafter measurements and nail plates are manually positioned in the joints of the jig. The nail plates are pressed into the timber by means of clamping arrangements and a press in the jig. In the jig, the plates can be only pressed to half the full depth, whereby the final pressing takes place usually in a finalizing roller. The jig can also be of such a construction that the nail plates are fixed and pressed first on the one side and then the rafter is turned around, i.e., turned over, and the plates of the other side are fixed so that fixing of plates underneath the timber members is not necessary.
There are a lot of problems with the presently used trussed rafter production methods as follows:
a) In all trussed rafter jigs, nail plates are fixed manually, so that the possibility of errors is remarkable, on the order of even up to 10 to 20 mm. These errors must be taken into account by making the plate bigger, which means a substantial increase of nail plate costs. Even though the plate size is made bigger, the final result is still unreliable, since an error in plate positioning is difficult to detect and control. PA1 b) The rafter assembly jig is expensive. If the plate is pressed by the direct method, i.e., usually by using a beam press or a C-press, the jig and the pressing element must be robust because of the great pressing force extended on the nail plate. If pressing is done by the roller method, measuring is largely carried out by a prefixing jig station and the final-fixing roller for the plates, as well as the conveyors between them, are expensive equipment. If the production is based on turning the trussed rafter, two separate jigs and plate positioning stages are needed, which makes the method complicated and for this reason, the method is not frequently applied. PA1 c) On starting the production of a trussed rafter batch, the jig must be installed, a form exactly similar to the trussed rafter found, and the positions of joints and plates determined. This is usually done by manual measuring, which is a time-consuming job. For jig setting, there are also many kinds of automatic measuring units and purely data controlled jigs have also been developed. However, jigs of this kind are expensive and generally applicable only to the production of standard type trussed rafters or the jigs do not carry out the setting completely but often many manual operations are still required, e.g., positioning of joints and plates must still be determined. PA1 d) The percentage of waste in production of trussed rafter timber members is remarkable, being on the order of 5% of all timber. This is mainly due to two reasons: firstly, cutting is usually done by four blade saws which always means some waste, because almost in every case a small bit has to be sawn off for timber member head formation, and secondly, the timber member length very seldom corresponds to the length of timber members to be sawn but is usually somewhat longer. This results in waste pieces that can not be used in the production. PA1 e) It is difficult to integrate the trussed rafter timber handling with automatic stress grading. This is due to the fact that in stress grading timber members move lengthwise and thus four blade cutting and stress grading cannot be combined economically. Present cutting saws are manual and it has not been possible to combine such saws with stress grading due to the slow movement thereof. PA1 f) Both manual and mechanical quality and stress grading methods presently used are inherently very uneconomical. Since the grading is based on the principle that in the design for each timber member the greatest stress is determined and, accordingly, the strength requirement for this timber member is set using the principle that the weakest stress point must correspond at least to the greatest stress. Timber strength is determined by defect, i.e., by the weakest point, and this strength is almost always quite small with respect to that of the overall timber member. It is typical that in a trussed rafter the stress is also very small, because in the trussed rafter dimensioning, the stress is a peak type moment load. Since there is very little probability that the decisive stress and the decisive defect or weakness will actually coincide, very high standards are set for timber quality in present methods. The effect of this fact is rather great in trussed rafters with dominating peak-type stress (in contrast to other constructions , e.g., beams and pillars). This is of great importance, because the timber itself is the biggest cost item in trussed rafter production.
A basically similar requirement of excessively high quality timber is related to timber wane and to the fact that such wane cannot be allowed in joints under the nail plate. Trussed rafter timber members could have substantial wane outside the joint area without any practical harm but this is not possible with present production methods which require full-edged timber.